Norman Reedus participates in "The Walking Dead" panel on Day 3 of Comic-Con International on Friday, July 19, 2103, in San Diego. (Photo by Jordan Strauss/Invision/AP)

SAN DIEGO (AP) -- The zombies are adapting on "The Walking Dead."

The fourth season trailer debuted at Comic-Con showed off a breed of stronger, savvier undead -- or "walkers," as they're called on the hit AMC zombie apocalypse series -- encroaching on the survivors at their prison enclave.

"It's going to get insane very quickly," new show runner Scott Gimple said.

"The Walking Dead" producers were joined by such cast members as Andrew Lincoln, Danai Gurira and Norman Reedus.

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In the footage shown at the pop-culture convention, the survivors' barricaded headquarters have been further fortified. However, increasingly more hordes are bombarding the gates, and they're not spreading out anymore.

"Yeah, the walkers are no threat at all this season," joked executive producer Gale Anne Hurd after panel moderator and "Talking Dead" host Chris Hardwick suggested the zombies were simply animals.

The clip also teased a catastrophic event inside the prison with multiple deaths. At the end of the trailer, Reedus' Daryl is shown driving a car and overhearing a voice on the radio whispering, "Those who arrive survive."

"The Walking Dead" creator Robert Kirkman said the villainous Governor played by David Morrissey will return in the fourth season but when, how and why will be a mystery that runs throughout the season.

"You've just gotta be on my side. If you're not on my side, you've gotta go," said Morrissey of his cutthroat character. "That's the deal with the Governor."

Several fans suggested to the panel what they'd like to see in the new season, including Rick and Michonne hooking up and Carl offing the Governor.